Bauxite is the basic raw material for almost all manufactured alumina compounds. In the course of production of aluminum compounds, most bauxite is refined to aluminum hydroxide by the Bayer process. The Bayer process involves hot leaching of bauxite with NaOH solution in pressure vessels to obtain supersaturated sodium aluminate solutions from which Al(OH).sub.3 is precipitated by seeding. Bayer process refineries share six common process stages: bauxite mining; raw material preparation; bauxite digestion; separation, washing, disposal of insoluble bauxite residue; aluminum hydroxide (trihydrate) precipitation; and calcination to anhydrous alumina. The process of separating bauxite residue solids from the supersaturated green liquor near its boiling point is known as "clarification".
In the clarification stage, the coarser solid particles are generally removed with a "sand trap" cyclone. To separate the finer solid particles from the liquor, the slurry is normally fed to the center well of a mud settler where it is treated with a flocculant composition that may be based on a variety of flocculating agents including starch, flour, polyacrylate salt polymer, acrylate salt/acrylamide copolymer, and/or water-soluble polymers containing pendant hydroxamic acid or salt groups. As the mud settles, clarified sodium aluminate solution, referred to as green liquor, overflows a weir at the top of the mud settling tank and is passed to the subsequent process steps. The sodium aluminate solution is generally cooled to enhance supersaturation and then seeded, e.g. with fine gibbsite seed from previous cycles to initiate precipitation of the desired endproduct Al(OH).sub.3.
The settled solids from the flocculation procedure, known as red mud, are withdrawn from the bottom of the mud settler and passed through a countercurrent washing circuit for recovery of sodium aluminate and soda. Aluminate liquor overflowing the settler may still contain significant amounts of suspended solids. This liquor is generally further clarified by filtration to give a filtrate that contains a very low level of suspended solids.
Alumina in relatively pure form is precipitated from the filtrate as alumina trihydrate crystals. The remaining liquid phase is returned to the initial digestion step and, after being reconstituted with additional caustic, is employed as a digestant of additional ore.
The suspended solids should be separated at a relatively fast rate if the overall Bayer process is to be efficient. Efficient removal of suspended solids from Bayer process streams has been a major challenge for many years. Among the methods of speeding up separation of suspended solids from process streams as well as effecting a cleaner separation of the constituents are those disclosed in U.S. Pat. No. 3,390,959, which employs polyacrylates as flocculants, and U.S. Pat. No. 3,681,012, which uses combinations of polyacrylates and starch in Bayer alumina recovery circuits. U.S. Pat. No. 4,083,925 discloses the use of polyacrylamide within the mud settler. U.S. Pat. No. 4,678,585 teaches that different stages in the Bayer alumina recovery circuit are advantageously treated with different flocculant compositions. U.S. Pat. No. 4,767,540 describes a process for removing suspended solids from Bayer alumina process streams by contacting and mixing a Bayer process stream with hydroxamated polymers. The hydroxamated polymers may be employed with anionic polyacrylate.
The prevailing practice where two or more different types of flocculant emulsions are to be used to clarify Bayer process streams has been to individually invert emulsions containing each of the polymer types in water prior to separately feeding them into the alumina plant process stream. This necessitates two or more polymer makeup/dissolution systems, with concomitant increased equipment, material, energy, labor, and other costs.
Thus, there is a need for improved methods for adding such flocculants to process streams. This invention provides various solutions to this problem.